Lasers, Radiation & Safety
Practical Skills - OCR A-Level Physics
Using Lasers in Physics
Key Definition
Coherent Light
Light with a constant phase difference between waves. Lasers produce coherent, monochromaticLight consisting of a single wavelength (and therefore a single colour). light, making them ideal for interference and diffraction experiments.
Light with a constant phase difference between waves. Lasers produce coherent, monochromaticLight consisting of a single wavelength (and therefore a single colour). light, making them ideal for interference and diffraction experiments.
- Lasers are essential for investigating wave properties such as interference and diffraction (e.g. Young's double-slit experiment).
- Lasers produce sharper and brighter interference patterns than regular lamps, giving clearer fringes for accurate measurement.
- Standard lamps emit light across all wavelengths with low intensity, resulting in dim, blurred fringes.
- Only Class 2 lasers (power output less than 1 mW) are permitted in school laboratories.
Laser Safety Precautions
- Never look directly at a laser beam or its reflection -- even brief exposure can cause permanent retinal damage.
- Never point a laser towards another person.
- Remove reflective objects (mirrors, watches, jewellery) before experiments to prevent accidental reflections.
- Wear appropriate laser safety goggles rated for the wavelength being used.
- Display "Laser On" warning signs outside the laboratory door.
- Always stand behind the laser so the beam travels away from you.
Ionising Radiation
Key Definition
Ionising Radiation
Radiation with enough energy to remove electrons from atoms, creating ions. Includes alpha particles, beta particles, X-rays, gamma rays, and UV radiation.
Radiation with enough energy to remove electrons from atoms, creating ions. Includes alpha particles, beta particles, X-rays, gamma rays, and UV radiation.
- Applications include: medical imaging (X-rays, CT scans), radiotherapy, sterilising equipment (gamma rays), and radioactive tracers.
- Excessive exposure can damage cells, cause mutations, and increase cancer risk.
Radiation Safety in the Laboratory
- Always handle radioactive sources with tongs or forceps -- never with bare hands.
- Keep sources in lead-lined containers when not in use.
- Point sources away from people and maximise distance from the source.
- Minimise handling time and store sources securely in designated areas.
- Workers wear radiation monitoring badgesBadges containing photographic film that darkens when exposed to radiation, indicating total exposure over time. that are regularly checked.
- Barriers absorb radiation: lead for gamma/X-rays, aluminium for beta, paper or air for alpha.
Geiger-Muller Tube
- A Geiger-Muller tubeA radiation detector that counts ionisation events. Radiation enters through a thin window, ionises gas atoms, and creates electrical pulses that are counted. is the most common radiation detector.
- Radiation enters through a thin window, ionises gas atoms inside, and creates a brief electrical pulse for each detection event.
- Activity is measured in becquerels (Bq) -- one becquerel equals one decay per second.
- Background radiationNatural radiation from cosmic rays, radioactive rocks, radon gas, food, and building materials. Must be measured and subtracted to find the true source activity. must be measured separately and subtracted from readings to find the true source activity.
- Allow sufficient counting time (typically 60 seconds or more) and record multiple readings to calculate means.
Common MistakeMEDIUM
Wrong: Forgetting to subtract background radiation from Geiger counter readings.
Right: Always measure background count rate first (without the source), then subtract it from all experimental readings to obtain the corrected count rate.
Right: Always measure background count rate first (without the source), then subtract it from all experimental readings to obtain the corrected count rate.